Facebook’s sagging public offering is bringing down values across Silicon Valley, from the mighty — like Twitter — to the lowly startup.

Twitter shares are down at least 15 percent on secondary markets, with the stock recently trading at $18, down from $21.

The decline hammered the valuation of the once-hot Twitter, pushing it down to about $8.7 billion from $10 billion, according to Sam Hamadeh, CEO of PrivCo, which tracks tech companies.

The private markets were a hotbed of speculative gambling on Facebook, which wound up disappointing investors when the company went public and the stock tanked.

Facebook is down over 30 percent from its initial public offering price of $38, closing yesterday at $25.87, after hitting a new low.

“I think [Facebook’s stock flop] will be particularly impactful on the late-stage and secondary markets where most of the IPO valuation speculation is happening,” wrote Fred Wilson of Union Square Ventures.

Wilson, an investor in Twitter, was responding in his blog to another warning that came from Paul Graham, a Silicon Valley star, who proclaimed that the days of easy money are over.

Facebook’s disastrous start as a public company threatens to chill fundraising for early-stage companies, Graham wrote: “No one knows yet how much. Possibly only a little. Possibly a lot, if it becomes a vicious circle.”

Graham is the founder of Y Combinator, an incubator for a select group of tech startups that are written $150,000 checks when they walk in the door, no questions asked.

DST Global, led by Graham’s friend Yuri Milner, parlayed a series of private Facebook investments, amounting to about $800 million, into a $1.75 billion IPO payday.